It has long been known that only a portion of the oil can be recovered from an oil-bearing subterranean formation as a result of the natural energy of the reservoir. So-called secondary recovery techniques are used to force more oil out of the reservoir, the simplest method of which is by direct replacement with another medium, usually water or gas.
“Water-flooding” is one of the most successful and extensively used secondary recovery methods. Water is typically injected, under pressure, into reservoir rock formations via injection wells. The injected water acts to help maintain reservoir pressure, and sweeps the displaced oil ahead of it through the rock towards production wells from which the oil is recovered. The water used in water-flooding may be a high salinity water, for example, seawater, estuarine water, a saline aquifer water, or a produced water (water separated from oil and gas at a production facility). By “high salinity” water is meant that the water has a total dissolved solids (TDS) content of greater than 20,000 ppmv, for example, greater than 30,000 ppmv.
Enhanced oil recovery (EOR) techniques can also be used. The purpose of such EOR techniques is not only to restore or maintain reservoir pressure, but also to improve oil displacement in the reservoir, thereby further reducing the residual oil saturation of the reservoir, that is, the volume of oil remaining in the reservoir.
Injection of an aqueous solution of a polymer (also known as polymer flooding) has been used as an EOR technique for over 60 years. Its application has been predominantly onshore in reservoirs, in which the oil is of relatively high viscosity.
Generally, polymer flooding consists of adding a water-soluble polymer to an injection water (aqueous base fluid), thereby providing an injection fluid having an increased viscosity and reduced mobility in the reservoir compared with the aqueous base fluid. Polymer flooding increases hydrocarbon, e.g. oil, recovery above secondary recovery (e.g. water flooding) mainly by improving microscopic and volumetric sweep efficiency. This can be especially beneficial, since in many reservoirs injection water is more mobile than oil so it tends to bypass lower permeability regions of the reservoir leaving behind significant volumes of oil. The difference between the mobility of the aqueous solution of the polymer and that of the oil in the reservoir is less than the difference between the mobility of the aqueous base fluid and the oil in the reservoir. This reduction in the mobility of the aqueous injection fluid relative to the mobility of the oil may lead to an enhanced oil recovery.
Normally, reservoirs having oil viscosities of 3 centipoise (cP) or greater may be considered suitable for polymer flooding, i.e. EOR using an aqueous solution or dispersion of a polymer.
It is also known that the use of a lower salinity injection water during water-flooding can increase the amount of oil recovered compared to the use of a higher salinity water. The low salinity water may be, for example, lake water, river water, a low salinity aquifer water, or a desalinated water.
It is also known that reducing the multivalent cation content of a lower salinity injection water can have an impact on the oil recovery.
Thus, International Patent Application No. WO2008/029124 teaches that oil recovery from a reservoir comprising a sandstone oil-bearing rock formation is enhanced (in comparison with injection of a high salinity water) when the injection water has a total dissolved solids (TDS) content in the range of 200 to 12,000 ppmv and the ratio of the multivalent cation content of the injection water to that of the connate water contained within the sandstone rock is less than 1.